The above fails out with a “Sequence does not respond to ‘:’” exception, like the ‘:’ is not viewed as an Operator after the “Bob Smith” Sequence.

Later, the “builder.io” script is able to create a new language structure using the “forward” method override, which allows for adding the generating code right in the same script. Is that possible to be done with the “phonebook.io” method?

Probably way too late, but for people reading the book nowadays and stumbling upon the same problem: https://www.ybrikman.com/writing/2012/02/07/seven-languages-in-seven-weeks-io-day-3/ has the answer: you have to put the operator-table-change into a separate file. The operator table is only loaded once at the beginning of the file, changing it will only affect further files, never the current file.